In a radio communication system using radio technology, a communication terminal realizes a network connection by exchanging various kinds of data by establishing synchronization with the synchronizing signal sent out from a radio base station.
FIG. 1 is a diagram showing one mode of a typical radio communication system.
The radio communication system shown in FIG. 1 includes terminal 1001, base station 1002 and network 1003.
Terminal 1001 is a communication terminal having a radio communication function. For example, a mobile phone and a mobile terminal can be mentioned.
Base station 1002 is a radio base station that connects terminal 1001 with network 1003.
Network 1003 is a communication network, for example, may be a WiMAX (Worldwide Interoperability for Microwave Access) network.
Here, data to be transmitted and received between terminal 1001 and base station 1002 have to be transmitted and received in synchronization with the aforementioned synchronizing signal. Accordingly, terminal 1001 needs to detect the time (temporal position) of transmission of the synchronizing signal transmitted from base station 1002.
In order for terminal 1001 to detect the synchronizing signal transmitted from base station 1002, it is common for a search to be performed by switching from one frequency to another, the received electric field of which is detected at terminal 1001.
FIG. 2 is a diagram showing a format of data frames transmitted and received between terminal 1001 and base station 1002 when network 1003 shown in FIG. 1 is a WiMAX network.
As shown in FIG. 2, the data frame transmitted and received between terminal 1001 and base station 1002 is configured of a preamble indicating the lead of the data frame, DL or downlink data, UL or uplink data and TTG/RTG or Gap time for absorbing propagation delay between terminal 1001 and base station 1002. Here, the preamble is used as the synchronizing signal for establishing synchronization between terminal 1001 and base station 1002. This data frame is transmitted and received between base station 1002 and terminal 1001 in a predetermined cycle (in a cycle of 5 ms in a WiMAX system).
FIG. 3 is a sequence diagram for illustrating a typical synchronizing signal searching process at terminal 1001 in the radio communication system shown in FIG. 1. In this case, description will be made taking an example where a synchronizing signal is transmitted from base station 1002 using frequency f2.
A synchronizing signal is periodically transmitted from base station 1002 using frequency f2.
First, at Step 11, a synchronizing signal search is performed at frequency f1 by means of terminal 1001.
Since no synchronizing signal is detected at frequency f1, a synchronizing signal search is performed by changing the frequency to f2 at Step 12.
Since the synchronizing signal transmitted from base station 1002 is transmitted using frequency f2, the synchronizing signal is detected in the search process at Step 12.
Then, at Step 13, connection to network 1003 via base station 1002 is completed using frequency f2 at which the synchronizing signal was detected.
However, when synchronizing signal Searches are sequentially performed by switching the frequency, since search processes need to be done for the frequencies at which no synchronizing signal exists, time is wasted for unnecessary searching processes so that it takes time to detect the synchronizing signal.
To address this problem, a technology exists that is used for identifying the frequency in accordance with the position of a communication terminal (mobile unit), by making the communication terminal store the correspondence between positional information represented by latitude and longitude and the information on the frequency used at that position (see JP2006-135955A, for example).
However, in the technology disclosed in JP2006-135955A, the frequency information associated with positional information represented by each latitude and each longitude needs to be stored in the communication terminal, hence there occurs the problem in which a large-scale storage area is needed for the communication terminal.
The object of the present invention is to provide a communication terminal and a connecting method and program for solving the above problems.
In order to attain the above object, the present invention resides in a communication terminal having a radio communicating function, comprising:
a GPS module that acquires positional information from a GPS satellite, indicating the position at which the communication terminal is present; and,
a controller which determines a range of correspondence information to be acquired, form among the correspondence information that makes the positional information indicating position identified by latitude and longitude correspondent to the frequency to be used at that position and that is stored in a server, based on the positional information acquired by the GPS module; and,
a receiver that acquires the correspondence information included in the range from the sever, wherein the controller, based on the positional information acquired by the GPS module and the correspondence information acquired by the receiver, identifies the frequency to be used between the communication terminal and a radio base station to be connected to the communication terminal.
Also, the present invention resides in a connecting method for connecting a communication terminal to a network through a radio base station, comprising the steps of:
acquiring positional information from a GPS satellite, indicating the position at which the communication terminal is present;
determining a range of correspondence information to be acquired, from among the correspondence information that makes the positional information indicating position identified by latitude and longitude correspondent to the frequency to be used at that position and that is stored in a server, based on the acquired positional information;
acquiring the correspondence information included in the range from the sever; and,
identifying the frequency to be used between the communication terminal and the radio base station to be connected to the communication terminal, based on the acquired positional information and the correspondence information included in the range.
Also the present invention reside in a program that causes a communication terminal having a radio communication function to execute:
a procedure of acquiring positional information from a GPS satellite, indicating the position at which the communication terminal is present;
a procedure of determining a range of correspondence information to be acquired, from among the correspondence information that makes the positional information indicating position identified by latitude and longitude correspondent to the frequency to be used at that position and that is stored in a server; based on the acquired positional information;
a procedure of acquiring the correspondence information included in the range from the sever; and,
a procedure of identifying the frequency to be used between the communication terminal and the radio base station to be connected to the communication terminal, based on the acquired positional information and the correspondence information included in the range.
As described heretofore, the present invention is constructed such that a communication terminal acquires positional information indicating the position at which the communication terminal is present, from a GPS satellite, determines a range of correspondence information to be acquired, from among the correspondence information that makes the positional information indicating position identified by latitude and longitude correspondent to the frequency to be used at that position and that is stored in a server, based on the acquired positional information, acquires the correspondence information included in the determined range, from the sever, and, identifies the frequency to be used between the communication terminal and the radio base station to be connected to the communication terminal, based on the acquired positional information and the correspondence information included in the range. It is therefore possible to easily shorten the time for a terminal to connect to a network and also to easily cut down the power consumption that is required for establishing a connection.